This invention relates generally to optical systems that employ holograms as the optical elements. More particularly, the present invention relates to a liquid-crystal display (LCD) that employs holograms to provide backlighting per operation in either a reflection mode or a transmission mode.
Modern reflective displays are intended for the image to be viewed with a source of light positioned xe2x80x9cbehind the backxe2x80x9d of the person viewing the display. When the angle of incidence of the ambient light source is oblique, i.e. xe2x80x9cin front of the viewerxe2x80x9d, the image appears to be blind with shade covering a part of the image.
It has been proposed to use reflection holograms for the improvement of the reflectance of the reflective display that contains a liquid-crystal panel combined with a holographic diffuser (positioned behind the panel) and a reflecting layer. However, such a holographic diffuser does not resolve the shade problem that occurs with light having an oblique angle of incidence. In addition, this approach does not provide the reflective mode for the whole spectrum of white light (for the color displays and for the black-and-white displays of the xe2x80x9cwhite paperxe2x80x9d type). For further details, refer to U.S. Pat. No. 5,659,408 issued to Wenyon.
Certain displays operate in the reflective mode and contain a dielectric backing with an applied polyamide insulating layer and an electrode system, which provide the diffuse reflection of light, a liquid-crystal layer, which changes its properties if the voltage is applied to the electrode system, and a coating equipped with a system of relevant transparent electrodes and given light-diffusing properties. The light is reflected directly from the bottom electrode of each liquid-crystal cell so that the display could operate in the reflective mode given a wide spectrum of incidence angles. It is quite different from the conventional external reflector which, in its turn, operates effectively in the narrow spectrum of incidence angles close to the perpendicular to (i.e., the normal angle with respect to) the reflective display plane. This type of reflective display does not operate in the transmission mode. Furthermore, the manufacturing technology germane to such a display is very different from the conventional and cheap technology of the transmission-based liquid-crystal displays.
Another weakness of this type of reflective display is its isotropism (i.e., even diffusion in all directions) when diffusion perpendicular to the display plane (i.e., toward the viewer of the display) would be preferred. In addition, the angles of reflection of light from the refractory electrode are symmetric to the angles of incidence with respect to the perpendicular to the display plane (i.e., they are specular). On the other hand, an asymmetric scheme wherein light incident at oblique angles (e.g., say from 30 to 70 degrees) is reflected at angles that are close to the perpendicular (e.g., +/xe2x88x9215 degrees with respect to normal angle) would be preferable.
It has also been proposed for an LCD display to employ a holographic diffuser for the backlighting of the liquid-crystal panels. Such a display contains a flat plate made out of a translucent material that is used as a waveguide, a reflective or transmissive hologram applied to the waveguide surface, a reflective layer applied to another surface of the waveguide, and a liquid-crystal panel operating in the transmission mode. For further details, refer to U.S. Pat. No. 5,724,111 to Mizobata et al.
As to this type of display, light from the source enters the waveguide at an end face of the waveguide where the source is located. The light being diffused all over the waveguide diffracts on the hologram, the outcoming light evenly illuminates the liquid-crystal panel cells with the parameters of the mentioned holographic diffuser (time interval, the angular spectrum of diffracted waves) being set so that the outcoming beams move in a direction sufficiently close to the perpendicular to the display plane.
Such display will not operate in a reflective mode. As the incidence angle of the outside beam which has passed through the liquid-crystal panel is not optimal, it is necessary for the beam to cover the whole path twice going through all the diffusions and absorptions in the optical system so that it could come back to the location of the liquid-crystal display even if the light becomes diffracted on the hologram. Hence, the operating effectiveness of such display in the reflective mode is practically zero both if the incidence is oblique or perpendicular (which is inconvenient because that geometry requires the light source be behind the viewer).
The present invention is directed to both a system and a method of manufacturing of a universal display that is reflective and transmissive, and produces a quality image. The system and method use simple construction and manufacturing technology. A holographic universal display according to the present invention uses reflective lighting for a liquid-crystal panel. The system comprises at least two waveguide holograms in a matched configuration that redirects incident light beams as diffracted beams so that the beams passing out of the display are targeted towards the viewer. That is to say, light beams that are incident to the display at large angles, over a wide range, with respect to normal are redirected into a set, narrow range of angles close to the required angle for optimal viewing. The required angle for optimal viewing is nominally perpendicular to the plane of the display (i.e., normal), but is optionally selectable (by design of the holograms) to be an angle varies from the strict perpendicular.
Optionally, a holographic universal display according to the present invention also operates in the transmission mode with the side illumination generated by the special light source optically connected with the waveguide.
A display manufactured using this method comprises a liquid-crystal panel with multiple electrically-controlled transmission cells, and an illuminator block. The illuminator block includes an optical waveguide element that is as thick as at least several light waves, a first hologram on a first surface of the liquid-crystal panel, and a second hologram on a second side surface of the waveguide element. A light source is optically connected with the waveguide.
Following the method according to the present invention the first hologram transforms the incident beams being at a large angle (more than 20 degrees) with the waveguide mode, i.e. the waves that exist only in the waveguide and are fully reflected internally at its boundary. The second hologram is intended for the reproduction by such a waveguide mode that is excited during the diffraction on the first hologram. As a result of such diffraction the second hologram transforms the waveguide mode into an outcoming light wave that reaches the viewer""s eye after passing through the liquid-crystal panel.
For holograms that are properly embodied according to the present invention, the incident light beam that has passed through a certain cell of the liquid-crystal panel is re-radiated, after the double diffraction by the holograms, so that it goes out of the display through the very cell through which it entered the panel. This hologram system compensates chromatic aberrations, i.e., dependence of the light wavelength on the diffraction angle and the illumination block used for the formation of the outcoming achromatic, i.e. white light.
This method enables utilization of liquid-crystal panels, including the color panels, both in the transmission and reflective modes, the reflective mode using the most comfortable configuration of the incident lightxe2x80x94above and in front of the viewer and the normal view of the reflected light. This method enables us to produce the achromatic reflected light and, accordingly, use this techniques in the color liquid-crystal displays.